Orthodontic appliances are devices used in the professional supervision, guidance and correction of a patient's malpositioned teeth. The many benefits of orthodontic treatment include the attaining and maintaining of a proper bite function, enhancement of facial aesthetics, and easier maintenance of dental hygiene. Orthodontic appliances are placed in mechanical engagement with the patient's teeth and apply gentle mechanical forces that gradually move the teeth toward corrected positions to achieve a proper bite (or occlusion).
A very common type of orthodontic treatment uses tiny slotted appliances called orthodontic brackets, which are adhesively attached to either the front or back surfaces of the patient's teeth. To move the teeth within an upper or lower arch, a resilient arch-shape wire (“archwire”) is mechanically engaged, or “ligated,” into the slot of each bracket. The ends of the archwire are generally captured in appliances called molar tubes, which are bonded to the patient's molar teeth. As the archwire slowly returns to its original shape, it acts as a track that guides the movement of teeth toward their desired positions. The brackets, tubes, and archwire are collectively known as “braces.”
Conventional brackets are ligated to the archwire with the help of opposing tiewings, which are cleat-like projections on the bracket body. After the archwire is placed in the archwire slot, either a tiny elastomeric “O”-ring ligature or a metal ligature wire is looped over the archwire and beneath the undercut portions of tiewings located on opposite sides of the archwire slot. By tightly encircling the undercut portions of the tiewings, the ligature (or ligature wire) can secure the archwire within the archwire slot of each bracket, while still allowing the archwire to slide longitudinally along the slot. Depending on the relative sizes and shapes of the archwire and the slot, it is possible to achieve a precise mechanical coupling between the two bodies. This enables the practitioner to control the position and orientation of each individual tooth in the arch.
Both of the ligating mechanisms above have certain drawbacks. For example, the frictional contact between O-ring ligatures and the archwire can increase resistance to archwire sliding within the slot. Moreover, the elastic properties of these ligatures can degrade over time, resulting in unpredictable sliding mechanics. While these ligatures can be made from translucent polymers for aesthetic treatment, these same ligatures also tend to stain in the presence of dark-colored foods and liquids. Ligature wire poses its own problems, since the process of tying and trimming the wire can be cumbersome and time-consuming for the orthodontic professional. Being made of metal, ligature wire is also considered non-aesthetic.
Self-ligating brackets present a solution to at least some of the above problems. These appliances generally use a clip, spring member, door, shutter, bail, or other ligation mechanism built into the bracket itself to retain the archwire in the slot, thereby obviating use of a separate ligature. Several advantages can derive from the use of these ligation mechanisms. For example, these appliances can decrease friction between the archwire and the bracket compared with appliances ligated with elastomeric ligatures, potentially providing faster leveling and aligning of teeth in early stages of treatment. Depending on the mechanism, these appliances can also simplify the installation and removal of an archwire, significantly reducing chair time for the treating professional. Finally, self-ligating brackets can provide better hygiene than conventional brackets, which use elastomeric ligatures and ligature wires that can trap food and plaque.
Aesthetic self-ligating appliances, particularly those made from ceramic materials, are also generally “passive” ligation devices. In passive ligation, the archwire is held captive within the slot but allowed to “float” freely within the archwire slot. Passive self-ligating brackets have a slot depth sufficiently large such that a continuous force seating the archwire into the slot is not exerted. Such a configuration can provide low friction between archwire and appliance but the freedom of movement within the archwire slot can compromise control. By contrast, in “active ligation,” the appliance imparts a continuous force urging the archwire toward the bottom wall or side wall of the slot. Active ligation can be desirable in some stages of treatment, particularly when using square and rectangular archwires, because “actively” seating these wires into the bracket slot can improve transmission of torque and rotational forces to the teeth. Still other brackets are engineered to be either active or passive, depending on the size and configuration of the archwire.